Switches that are responsive to temperature changes, commonly known as thermostats or cold controls, are used in refrigeration appliances, such as refrigerators and freezers, to control the temperatures therein. These thermostats regulate the switching cycle of the refrigeration compressor in response to the temperature of the air contained at some location within the appliance. When the temperature exceeds a certain "turn-on" point, the switch contacts are closed and the compressor is switched on to cool the appliance. When the temperature drops below a certain "turn-off" point, the switch contacts are opened and the compressor is switched off. The temperature then begins to rise, and the refrigeration cycle begins again. Examples of thermostats for refrigeration appliances are set forth in U.S. Pat. No. 2,795,674 issued to Grimshaw, U.S. Pat. No. 3,096,419 issued to Howell, and U.S. Pat. No. 4,937,549 issued to Kelly et al. All of these patents are assigned to the General Electric Company, the assignee of the present invention, and their disclosure is expressly incorporated herein by reference.
One specific type of refrigeration thermostat is often referred to as a "constant-on" thermostat. This type of switch is constructed to turn the compressor on at a constant, preset turn-on temperature. However, the compressor turn-off temperature is selectively adjustable by the user. Such constant-on thermostats are often used in "off-cycle defrost" refrigeration units, wherein the defrosting of the evaporating unit is initiated after each cooling cycle. The constant turn-on temperature is usually preset within the thermostat to be several degrees above freezing, for example, 36.degree. F. In this way, the frost which accumulates during each cooling cycle will melt away when the temperature of the evaporating unit is permitted to rise up to the compressor turn-on temperature. In such an off-cycle defrost refrigeration unit, it is necessary to prevent the user from manually adjusting the compressor turn-on temperature, since huge amounts of frost would build up on the evaporator if the user were to adjust the compressor turn-on temperature to be below 32.degree. F.
Accordingly, when the user adjusts the temperature control knob of a refrigeration appliance having a constant-on thermostat, only the compressor turn-off temperature is varied. The temperature differential, i.e., the difference in temperature between turn-on and turn-off of the compressor, is therefore also variable. However, the temperature control knob has no substantial effect upon the compressor's predetermined turn-on temperature. Hence, in a constant-on, variable-differential thermostat, the contact-closing temperature, i.e., that which is required to turn the compressor on, always remains the same, while the manual turning of the temperature control knob varies the contact-opening temperature to turn the compressor off.
In order to allow the user to vary the temperature at which the switch contacts will open, i.e., the compressor turn-off temperature, prior refrigeration thermostats, such as that shown in U.S. Pat. No. 3,096,419 to Howell, have utilized what is known as the variable-force technique. In the Howell device, a contact-operating lever is pivotally mounted to open and close the switch contacts when the lever is moved in opposite directions between two fixed-position stops. A snap-action toggle spring is in continuous engagement with the contact-operating lever to bias the lever toward one of the two positions. To selectively adjust the temperature at which the contacts will open, a rotatable cam is connected to the temperature control knob. Rotation of this cam varies the force that is applied to the operating lever by a special biasing spring. This changes the force which must be applied to the operating lever by the temperature-responsive bellows and, consequently, the temperature at which the bellows can apply sufficient force to move the operating lever to its other position to open the switch contacts and turn-off the compressor.
In order to manufacture constant-on thermostats utilizing the variable-force technique, it has been necessary to implement a relatively complex and intricate network of springs, levers, and cams. Such variable-force designs for the temperature control mechanism have a temperature adjustment range which is relatively limited and somewhat imprecise. Moreover, most variable-force thermostats have a limited useful life due to the electrical arcing characteristics of the switch contacts being controlled by the contact-opening mechanism. Furthermore, the complex structure of such thermostats increases the overall cost of the device and decreases its reliability.
A need, therefore, exists for a refrigeration thermostat of simpler construction, having wider temperature range capabilities, better arcing performance, and better reliability.